Measuring tension applied to a chain has been performed by attaching a sensor consisting of a small-sized strain gauge to at least one position of the chain as a portion to be measured. The sensor and an analyzer are connected by a wire. Further, a tension measuring method for use in an operating chain which employs transmitting a sensor output by radio and receiving the power by a receiver has been proposed [for example see Japanese Laid-Open Publication No. 2006-71452 (Japanese Patent Application No. 2004-255168)].